Direct Measurement of Grain-Boundary Sliding in Forsterite Bicrystals

Olivine is the most abundant mineral in Earth’s mantle, and its rheological behaviour is likely to control upper-mantle deformation. While the rheological behaviour of olivine is widely studied, relatively little is known about the behaviour of individual olivine grain-boundaries. There is a pressing need to advance our understanding of their physical and chemical properties. Forsterite bicrystals, synthesized by direct bonding of highly polished single crystals at high temperature, were tested in a creep apparatus to investigate sliding along a single planar grain-boundary at high temperature (1300°C and 1400°C). Prior to deformation, the lateral surfaces of the bicrystals parallel to the shear direction were polished, and fiducial markers were scribed perpendicular to the grain-boundary trace to track grain-boundary sliding. Bicrystals were deformed in shear between two polycrystalline alumina pistons or two single crystal forsterite pistons, at 1 atm, with applied resolved shear stresses ranging from 1 to 30 MPa. Post-deformation microstructural analysis using a scanning electron microscope (SEM) shows discrete offsets of fiducial markers, which is the first direct evidence of grain-boundary sliding in olivine bicrystals. These results establish that the studied grain-boundaries are significantly weaker than crystal interiors, and that, crucially, grain-boundary sliding is controlled by the crystallography of crystal interiors and is favoured in a direction nearly parallel to the weakest slip direction in both crystals of the bicrystal.  The measured effective grain-boundary viscosities fit well theoretical models of a dislocation grain-boundary sliding mechanism and are higher than measurements inferred from attenuation. This evidence may highlight the important role of boundary dislocations in accommodating grain-boundary sliding in large grain sizes. These new results indicate that grain-boundary sliding in olivine could play a crucial role in the development of crystallographic preferred orientation and the resulting seismic anisotropy in the upper mantle and should therefore be accounted for in geodynamic models of Earth’s interior.